The micromachined flow sensors are miniaturized mechanical and electro-mechanical devices. They were investigated for their superior characteristics compared to conventional ones due to their small size, increased sensitivity, fast response, and cost effectiveness. Since flow measurement is a classical field of measurement technology, the micromachined flow sensors have found many important applications in several fields, including automotive and aerospace applications, meteorological stations, industrial control equipment, medical equipment, etc. The growing market of the micromachined flow sensors requires a lot of research and development of this new technology and its use for fabrication of flow sensors.
Different types of micromachined flow sensors were published in the literature, most of them being of the thermal type and composed of suspended membranes on top of a cavity, on which the active elements of the sensor are fabricated. These structures offer optimum thermal isolation by the air under the membranes.
Toyota Central Research and Development Laboratories developed a micromachined flow sensor based on the conventional hot-film anemometry. The sensor chip has two platinum thin-film resistors which are used as a heating element and a fluid temperature sensing element. The resistors are located in the centre of the oxidized porous silicon diaphragm and the rim of the chip, respectively. (Tabata, O., Inagaki, H., Igarashi, I. and Kitano, T., Fast, Proceeding of the 5th Sensor Symposium, The Institute of Electrical Engineers of Japan, Japan, 1985, pp. 207-211).
It has been shown that porous silicon oxidation causes marked wafer warpage, breaking up membrane during their release while etching Si from the back side, and formation of membranes with a blister-like shape immediately after the cooling step from the oxidation temperature to room temperature.
Delft University of Technology (Netherlands) manufactured a flow sensor for measuring flow rate and flow direction was developed. In these sensors, thermopiles consisting of silicon-aluminum coupling in a two-dimensional arrangement are in use. The heater resistor is located on a suspended thin silicon plate for improvement of the heat isolation. (Verhoeven H. J. and Huijsing, J. H., An integrated gas flow sensor with high sensitivity, low response time and pulse-rate output. Sensors and Actuators A, 1994, 41/42, 217-220.)
It is not common to use a suspended silicon membrane as thermal insulation. Since the thermal conductivity of silicon is relatively high, the improvement of the heat isolation is limited.
D. N. Pagonis et al. provided a micromachined flow sensor using a manufacturing technique for forming an air cavity below the porous layer to increase the thermal isolation efficiency. Both porous silicon and the cavity underneath are formed during the same electrochemical process in two steps: in step 1 the current density used is below a critical value, and in step 2 it is switched to a value above the critical current for electropolishing. In this way, porous silicon is formed first, followed by the formation of the cavity underneath. (D. N. Pagonis, A. G. Nassiopoulou, and G. Kaltsas, Porous silicon membranes over cavity for efficient local thermal isolation in Si thermal sensors, Journal of The Electrochemical Society, 151(8) H174-H179 (2004))
Unfortunately, the created porous silicon layer is easy to detach from the silicon substrate. To overcome this problem an n-type silicon bar is created in a p-type silicon substrate by thermal diffusion. The porous silicon layer is created in the p-type region surrounded by the n-type silicon bar and supported not only by the masking layer but also by the n-type silicon bar. In this process, a P implantation dose of 2 by 10 sup.16/square cm is required. In addition, three consecutive oxidation steps are followed and the oxide formed in each oxidation is removed. This process is not compatible with a CMOS process. The obtained PS membrane is only 10 um-thick, it is too fragile for most of flow measurements.
Chang Liu et al. proposed a micromachined flow sensor consisting of a suspended silicon-nitride diaphragm located on top of a vacuum-sealed cavity. A heating and heat-sensing element, made of polycrystalline silicon material, resides on top of the diaphragm. The underlying vacuum cavity greatly reduces conductive heat loss to the substrate and therefore increases the sensitivity of the sensor (Chang Liu, Member, IEEE, Jin-Biao Huang, Zhenjun (Alex) Zhu, Fukang Jiang, Steve Tung, Yu-Chong Tai, and Chih-Ming Ho, A Micromachined flow shear-stress sensor based on thermal transfer principles, JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 8, NO. 1, MARCH 1999)
A drawback is that the fabricated sensor is also sensitive to changes of ambient pressure because the suspended silicon-nitride diaphragm is flexible and the heavily doped polysilicon serves as a piezoresistive element.
The U.S. Pat. No. 6,378,365 (Micromachined thermal flowmeter having heating element disposed in a silicon island; by Tu) revealed a micromachined thermal flowmeter comprising characteristically at least one crystal silicon island jutted into the flow of a fluid to be determined which are embedded in an elastic low thermal conductivity layer supported by a rigid low thermal conductivity plate having a heating element and a pair of thermal sensing elements formed therein.
This U.S. patents provided micromachined flowmeters that can work robustly in a dirty flow fluid without being disrupted for improved work efficiency.
The U.S. Pat. No. 6,139,758 (Method of manufacturing a micromachined thermal flowmeter; by Tu) teaches a method of manufacturing a micromachined thermal flowmeter. The major manufacturing steps comprise forming an n-type region(s) in a p-type silicon wafer, forming heating and temperature sensing devices in the n-type region(s), converting the n-type region(s) into porous silicon by anodization in a HF solution, bonding the silicon wafer onto a glass plate using a polyimide layer as an adhesive layer, removing the porous silicon in a diluted base solution, and coating the heating and temperature sensing devices with a corrosion-resistant and abrasion-resistant material.
This U.S. patents developed a thermal isolation technology based on selective formation of a porous silicon layer in a silicon substrate.
Compared to the above mentioned micromachined flow sensors, the vacuum-cavity-insulation flow sensor in the current invention disclosed will present superior properties in many aspects including easiness of fabrication, perfection of thermal isolation, strength of membrane structure, and lower cost of manufacturing.